A. Field of the Invention
The invention generally concerns the production of carbon monoxide, sulfur dioxide, and carbonyl sulfide from the reduction of carbon dioxide by elemental sulfur. The produced carbon monoxide can then be converted into synthesis gas (syngas) and other valuable chemicals, while the other reaction products can be used to produce additional economically viable chemicals (e.g., both sulfur dioxide and carbonyl sulfide can be used to produce fertilizers).
B. Description of Related Art
Carbon dioxide is a relatively stable and non-reactive molecule when compared with carbon monoxide. Carbon monoxide is more interesting in this respect, as it can be used to produce several downstream chemical products. For instance, syngas (which includes carbon monoxide and hydrogen gases) is oftentimes used to produce chemicals such as methanol, tert-butyl methyl ether, ammonia, fertilizers, 2-ethyl hexanol, formaldehyde, acetic acid, and 1-4 butane diol.
Syngas can be produced by common methods such as methane steam reforming technology as shown in reaction equation (1), partial oxidation of methane as shown in reaction (2), or dry reforming of methane as shown in reaction (3):CH4+H2O→CO+3H2 ΔH298K=206 kJ  (1)CH4+O2→CO+2H2 ΔH298K=−8 kcal/mol  (2)CH4+CO2→2CO+2H2 ΔH298K=247 kJ  (3)While the reactions in equations (1) and (2) do not utilize carbon dioxide, equation (3) does. Commercialization attempts of the dry reforming of methane have suffered due to high energy consumption, catalyst deactivation, and applicability of the syngas composition produced in this reaction. Equation (4) illustrates the catalyst deactivation event due to carbonization.CH4+2CO2→C+2CO+2H2O  (4)
Other attempts to convert carbon dioxide into carbon monoxide include the catalyst reduction of carbon dioxide using hydrogen as shown in equation (5).CO2+H2→CO+H2O ΔH=10 kcal/mol  (5)This process, which is also known as a reverse water gas shift reaction, is mildly endothermic and takes place at temperatures at about 450° C. However, commercialization of this process suffers from the hydrogen availability. In particular, hydrogen is relatively expensive to produce and isolate. Thus, the present costs and sources of hydrogen are not favorable on a commercial scale to convert CO2 to CO per equation (5).
While other attempts have been made to produce carbon monoxide from carbon dioxide, these attempts have also proven to be inefficient. For instance, U.S. Pat. No. 1,793,677, utilizes carbonaceous fuel (e.g., coal), carbon dioxide, and oxygen to produce a mixture consisting of carbon monoxide together with small quantities of carbon dioxide and sulfurous anhydride. The sulfurous anhydride is a by-product due to the minimal amounts of suphur present in the coal. In particular, carbon dioxide and oxygen are passed over the carbonaceous fuel at temperatures greater than 1000° C. The main source of carbon in this reaction is coal rather than carbon dioxide. Carbon acts as a reducing agent which reduces carbon dioxide and oxygen to carbon monoxide. Also, both oxygen and carbon dioxide acts as oxidizing agents. Therefore, the primary reactants are carbon, oxygen and carbon dioxide, and sulfur is a secondary reactant as its concentration in coal is very low. Sulfur is oxidized to sulfurous anhydride by either oxygen or carbon dioxide. Ultimately, the carbonaceous fuel source as well as the reaction temperatures adds cost and complexities to producing carbon monoxide.
Still further, Asadi et al., in Nature Communications 5, 2014, Vol. 5 pp. 4470, describes an electrochemical reduction of carbon dioxide using a molybdenum disulfide as a catalyst at 54 mV in an ionic liquid to produce carbon monoxide and hydrogen. The complexity and additional components needed to drive this reaction also results in an inefficient process that is not commercially viable for producing carbon monoxide.